Download Current Distribution in Parallel LED Strings Application Guide

Application Guide
Current Distribution in Parallel LED Strings
Introduction
Many LED applications require more than one LED
in a system. When multiple LEDs are used in a
system, the LEDs can be arranged in series,
parallel, or series- parallel configurations. Due to
regulatory requirements (especially in Class II
environments) there is always a limitation as to how
many LEDs can be connected in series. One of the
standard practices to overcome this issue is to
have multiple parallel strings of series LEDs that
are driven from a single constant current source.
This Application Guide describes the issues and
solutions that may be encountered when multiple
strings are driven from a constant current source
(CC source).
Series- Parallel configuration
In LEDs, Vf is the threshold voltage at which an
LED starts to turn on. Due to manufacturing
tolerances and material properties, Vf can vary
between LEDs even if they are from the same
wafer. Depending on the technology used, the
range in difference can be a few millivolts to up to a
volt. When multiple LEDs are connected in series,
this Vf mismatch between LEDs adds up and
becomes
significant.
In
series-parallel
configuration, if the string voltage is significantly
different between stings, the current through each
string will vary (become imbalanced) when a single
constant current source is used. Figure 2 shows
what the string currents would be if one of the
strings has ~3V (caused by a shorted LED) less
than the other two strings.
In series-parallel configuration, there are multiple
strings of series LEDs. It is always better to limit the
number of LEDs in series to minimize total Vf
(threshold voltage) mismatch between strings. It is
recommended to have 4 to 6 LEDs in a series so
the difference in voltage between strings is not
significant.
Figure 2. String Voltage vs string current
Figure 1. Series-Parallel configuration
Difference of Vf in LEDs
June, 2011
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Application Guide
Impacts of imbalanced string currents
If all of the strings in a series-parallel configuration
don’t have the same current going through them,
there will be a number of issues that have to be
dealt with during the design process. Imbalanced
string current will result in lowering the overall
system efficacy and this can be significant,
depending on how wide the difference in string
current is. It will also play a major role in LED
lifetime and thermal management. Moreover, if the
string current is considerably different between
strings, the light output from each string would be
different (Figure 3) and this difference will be visible
to the naked eye. This is not desirable in a system
with multiple LED strings. Displayed in Figure 3, the
string with lower total Vf, due to a shorted LED, has
lot more current going through it and a very small
portion of the total current is split between the other
two strings. Since Flux vs Current is not linear in
LEDs (Figure 4), the system efficacy will be
reduced when there is imbalanced string current.
Also, note that the two strings with identical Vf have
same current going through them. This can be
witnessed by looking at the brightness of the LEDs
in those two strings.
Figure 4. LED Current Vs Flux of OSLON LED
Impact on efficacy
With imbalanced string currents, and due to the fact
that efficacy of an LED degrades with increasing
current, the overall system efficacy of a system with
multiple LED strings would be lower. Figure 5
shows the difference in efficacy with balanced and
imbalanced string currents. In this example,
OSLON LEDs from OSRAM Opto Semiconductors
are used. The flux/light output from the OSLON
LEDs at 350 mA is set to be 100 lm. The junction
temperature of the LEDs are assumed to be room
temperature to eliminate temperature dependency
on flux and for ease of calculation. Circuit
configuration for the example would be the same as
in Figure 1. All the values used in the calculation
are obtained from the datasheet that is available on
Figure 3. Representation of Figure 2 in real
application
June, 2011
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Application Guide
the web site.
http://catalog.osramos.com/jsp/download.jsp?rootPath=/media/&name=LUW_CP7
P_Pb_free.pdf&docPath=Graphics/00052189_0.pdf&url=/medi
a//_en/Graphics/00052189_0.pdf
Also required for the calculation is the I-V curve
(Figure 5) for OSLON LEDs.
The efficacy of the circuit without considering
electrical, thermal, and optical losses would be:
1800lm/20.16W = 89.3lm/W
With imbalanced current (per current distribution in
Figure 2), the total lumen would be:
(1.75*100*6)+(0.54*100*12)= 1698lm
(as per Figure 4, flux at 695mA is ~1.75 times the
flux at 350mA and flux at 177mA is ~0.54 times the
flux at 350mA)
The total power in this case would be:
(0.177*12*3.05) + (0.695*6*3.45) = 20.86W
(per Figure 5, Vf at 177mA is ~3.05V and Vf at
695mA is ~3.45V)
The efficacy in this case would be:
1698lm/ 20.86W = 81.38 lm/W
One can easily see the difference of ~8lm/W
between the two cases. The difference of 8lm/W is
significant and should be taken into consideration
during the design and development stages. It
should be noted that the 8lm/W difference is due to
the fact that the difference in string current, in this
case, is significantly wider. If the difference in string
current is narrower, then the loss of efficacy would
be lower. The wider range string current is chosen
in this example to easily explain the efficacy impact
to the reader.
How to solve the issue?
Figure 5. Current Vs Voltage of OSLON LED
With balanced LED strings, the total lumen would
be:
18*100 = 1800lm
(there are 18 LEDs in total and 100lm from each
LED)
Total power in this case would be:
3.3*0.35*18 = 20.16W
(typical Vf for Oslon is 3.2V)
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There are multiple ways to address this issue. One
of them is to use individual constant current
sources for each string. This solution is costly and
adds more components to the driver circuitry. As a
result of adding passive and active components,
more failure modes are introduced and reliability of
the driver is also impacted to a certain degree. In
some cases, the space requirement may be a
factor as this solution requires considerably more
space for the driver and control circuitry. Another
Application Guide
way to address this issue is to use a current mirror
to match the string currents. This solution would be
much less expensive compared to individual
current sources and uses very few components. At
minimum, each string would require only one
transistor, put in a current mirror form, to match
current between the strings. As with any other
solutions, this also has some dis-advantages. One
of them is the reference current to the current
mirror. The way it works in this case is that the
reference current to the current mirror is set by one
of the strings and current to the other strings are
matched to the reference current. If the reference
string has lower voltage compared to the other
strings, then the current mirror will not work as
expected. This can be solved by inserting a resistor
in the reference string such that the reference string
has slightly higher voltage compared to all other
strings. The other issue is the dependency on the
Vbe (base to emitter voltage of a transistor) of each
transistor in the current mirror circuit. This issue
can be addressed by selecting reasonably matched
Vbe on the transistors. To reduce dependency on
the Vbe, a small value resistor can be inserted at
the emitter. However, it will add cost and may not
be required for the specific application that is being
considered. The circuit in Figure 2 is simulated with
current mirror in place and shows the current
distribution in each string.
With the addition of the current mirror in place the
string currents are now matched within 3-5%
between strings. However, note that the reference
string doesn’t have the lowest total Vf. As
mentioned before, you may run into issues when
the reference string has lower total Vf compared to
other strings, Figure 7 illustrates this situation.
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Figure 6. Current distribution with current mirror in
place.
Application Guide
(with added resistor in the reference string) in place.
Figure 7. Current distribution with current mirror
(reference current string has lower voltage) in place.
The addition of a resistor resolved the issue and
the string currents are matched within 2% (Figure
9).
The current mirror doesn’t do any good in this case,
this is due to the fact that the reference string has
lower voltage compared to the other two strings. To
address this issue, a resistor can be inserted in the
reference strings so that the reference string has
slightly higher voltage compared to the other
strings. There will be an efficacy loss due to the
resistor depending on the value of the resistor,
which is dictated by how wide the expected voltage
difference between strings is and how well the
string current needs to be matched. Figure 8 shows
simulation results with the addition of a resistor in
the reference string.
Figure 9. Representation of Figure 8
Figure 8.
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Current distribution with current mirror
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Application Guide
Even without any technical understanding, one can
easily see how the current mirror improved the
string currents, and therefore the light output from
each string, by comparing Figure 3 and Figure 9.
Below is a side-by-side view of the pictures for
comparison purpose.
Figure 10. Side-by-Side view (with and without
current mirror circuit)
Things to remember when designing a
current mirror
1. The transistors should be chosen such that
Vbe of all the transistors are reasonably
matched.
2. Adding a very small resistor at the emitter
pins of the transistors will also help to
reduce Vbe mismatch.
3. The selection of resistor should serve the
purpose and minimize efficacy losses.
4. It is recommended to have all transistors
operating at the same temperature to
ensure or minimize variations due to
thermal characteristics of transistors.
5. It is also possible to have one more
transistor and shut off the reference string, if
the current through the reference string
exceeds a set value.
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Conclusion
A well designed current mirror in a series-parallel
LED configuration can match the string currents to
within 3% to 5%. Since the passives used in a
current mirror are very minimal and inexpensive,
the added cost is almost negligible. Furthermore, if
designed properly, efficacy losses due to the added
components are insignificant.
Application Guide
Author: Muhinthan Murugesu, Applications
Engineer
About OSRAM Opto Semiconductors
OSRAM is part of the industry sector of Siemens
and one of the two leading lighting manufacturers in
the world. Its subsidiary, OSRAM Opto
Semiconductors GmbH in Regensburg (Germany),
offers its customers solutions based on
semiconductor technology for lighting, sensor and
visualization applications. OSRAM Opto
Semiconductors has production sites in
Regensburg (Germany) and Penang (Malaysia). Its
headquarters for North America is in Sunnyvale
(USA), and for Asia in Hong Kong. OSRAM Opto
Semiconductors also has sales offices throughout
the world. For more information go to www.osramos.com.
All information contained in this document has
been checked with the greatest care. OSRAM
Opto Semiconductors GmbH can however, not
be made liable for any damage that occurs in
connection with the use of these contents.
OSRAM Opto Semiconductor GmbH makes no
representations and warranties as to a possible
interference with third parties' intellectual property
rights in view of products originating from one of
OSRAM Opto Semiconductor GmbH's partners, or
in view of products being a combination of an
OSRAM Opto Semiconductor GmbH's product and
a product of one of OSRAM Opto Semiconductor
GmbH's partners. Furthermore, OSRAM Opto
Semiconductors GmbH cannot be made liable for
June, 2011
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any damage that occurs in connection with the use
of a product of one of OSRAM Opto Semiconductor
GmbH's partners, or with the use of a combination
of an OSRAM Opto Semiconductor GmbH's
product and a product of one of OSRAM Opto
Semiconductor GmbH's partners.
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